System and method for adjusting phase offsets

ABSTRACT

Systems and methods that adjust phase offsets are provided. In one embodiment, a phase detector may include, for example, a delay block coupled to a first exclusive OR (XOR) gate via a first delay element; a first sequential device coupled the first XOR gate via a second delay element; a second sequential device coupled to the first sequential device and coupled to a second XOR gate via a third delay element; and a third sequential device coupled to the second sequential device and coupled to the second XOR gate via a fourth delay element.

RELATED APPLICATION

[0001] This application is a continuation-in-part of U.S. patent application Ser. No. 09/955,693 entitled “Linear Phase Detector for High-Speed Clock and Data Recovery” and filed on Sep. 18, 2001.

INCORPORATION BY REFERENCE

[0002] The above-referenced United States patent application is hereby incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

[0003] High-speed data communication relies heavily on clock and data recovery (CDR) circuits. CDR circuits extract clock information from an incoming data signal (e.g., an incoming data stream) and employ the extracted clock signal to regenerate the data signal, thereby reducing noise from the original data signal and preventing noise accumulation along the communication lines.

[0004] A CDR circuit employs a full-rate phase-lock loop. When the loop is locked, the CDR circuit should generate a clock signal with the exact frequency of the incoming data signal and fix the phase relationship between the generated clock signal and the incoming data signal. In a conventional phase detector, the conventional lock position is such that the latching clock edge is at the center between the data transitions.

[0005] However, optimal data regeneration does not always occur at the conventional lock position of a conventional phase detector. For example, different asymmetries and jitter may occur in the data signal, especially if the data signal is propagating long distances. In addition, the data signal may be further degraded due to the severe dispersion and distortion that may occur in the particular communications medium (e.g., fiber, wire, cable, air, etc.).

[0006] Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of ordinary skill in the art through comparison of such systems with the present invention as set forth in the remainder of the present application with reference to the drawings.

BRIEF SUMMARY OF THE INVENTION

[0007] Aspects of the present invention may be found in, for example, systems and methods that adjust phase offsets. In one embodiment, the present invention may provide a phase detector. The phase detector may include, for example, a delay block coupled to a first exclusive OR (XOR) gate via a first delay element; a first sequential device coupled the first XOR gate via a second delay element; a second sequential device coupled to the first sequential device and coupled to a second XOR gate via a third delay element; and a third sequential device coupled to the second sequential device and coupled to the second XOR gate via a fourth delay element.

[0008] In another embodiment, the present invention may provide a system that adjusts a locked phase offset between a data signal and a clock signal. The system may include, for example, a phase detector in which one or more data paths used in generating an error signal or a reference signal comprise a respective delay element; a loop filter coupled to the phase detector; and a voltage controlled oscillator coupled to the loop filter and an output of the voltage controlled oscillator coupled to an input of the phase detector.

[0009] In yet another embodiment, the present invention may provide a method that adjusts a locked phase offset between a data signal and a clock signal. The method may comprise, for example, adjusting a time delay in a respective delay element disposed in each data path of a phase detector.

[0010] In yet another embodiment, the present invention may provide a method that adjusts a locked phase offset between a data signal and a clock signal. The method may comprise, for example, adjusting a first time delay in a first data path of a phase detector; and adjusting a second time delay in a second data path of the phase detector

[0011] These and other features and advantages of the present invention may be appreciated from a review of the following detailed description of the present invention, along with the accompanying figures in which like reference numerals refer to like parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 shows an embodiment of an optical receiver according to the present invention.

[0013]FIG. 2 shows an embodiment of a clock and data recovery (CDR) circuit according to the present invention.

[0014]FIG. 3 shows a diagram illustrating an embodiment of a phase detector according to the present invention.

[0015]FIG. 4 shows a diagram illustrating an embodiment of a phase detector according to the present invention.

[0016]FIG. 5 shows a diagram illustrating an embodiment of a delay element according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0017] Some aspects of the present invention may relate to systems and methods that adjust phase offsets, for example, in phase locked loops. Some embodiments according to the present invention may find application in use with, for example, phase detectors or clock and data recovery circuits.

[0018]FIG. 1 shows an embodiment of an optical receiver according to the present invention. Although illustrated with respect to an optical receiver, the present invention need not be so limited. The optical receiver 100 may include, for example, a photodiode 110, a first resistor 120, a pre-amplifier 130, an amplifier 140, a DC offset correction circuit 150, a second resistor 160, a clock and data recovery (CDR) circuit 170 and a link and data detect (LDD) circuit 180. The CDR circuit 170 may include, for example, two outputs 190, 200. The first output 190 may provide, for example, a data signal; and the second output 200 may provide, for example, a clock signal. The LDD circuit 180 may include, for example, two outputs 210, 220. The first output 210 may provide, for example, a squelch signal; and the second output 220 may provide, for example, a valid link signal.

[0019] An optical data path 230 may be coupled to the optical receiver 100 via the photodiode 110. The photodiode 110 and the first resistor 120 (e.g., a sensing resistor) may be in series and coupled between electrical ground and a voltage source VCC. A node between the photodiode 110 and the first resistor 120 may be coupled to an input of the pre-amplifier 130. An output of the pre-amplifier 130 may be coupled to a first input of the amplifier 140. An output of the amplifier 140 may be coupled to respective inputs of the DC offset correction circuit 150, the CDR circuit and the LDD circuit 180. The DC offset correction circuit 150 may be coupled to a second input of the amplifier 140 via a second resistor 160. The CDR circuit 170 and the LDD circuit 180 may also be directly coupled to each other.

[0020] In operation, an optical data signal may be carried by the optical data path 230. The optical data signal may be received by the photodiode 110 which may generate a current as a function of the received optical data signal. The generated current may result in a particular voltage across the first resistor 120. The voltage may be amplified by the pre-amplifier 130 and amplifier 140. Offsets in the amplified voltage may be, for example, reduced by the DC offset correction circuit 150 which may provide a feedback loop back to the amplifier 140 via the second resistor 160. The output of the amplifier 130 may drive the CDR circuit 170 and the LDD circuit 180. The CDR circuit 170 may extract a clock signal from the amplified data signal and provide the clock signal on the second output 200 of the CDR circuit 170. The CDR circuit 270 may also use the clock signal to retime the data for output on the first output 190 of the CDR circuit 170. If the LDD circuit 160 senses either a data or link signal from the amplifier 140, then a valid link signal may be asserted on the first output 210 of the LDD circuit 160. If the LDD circuit 160 senses a data signal from the amplifier 140, a receive squelch signal may be deasserted on the second output 200.

[0021]FIG. 2 shows an embodiment of a CDR circuit according to the present invention. The CDR circuit 170 may include, for example, a data retimer 240, a phase detector 250, a frequency detector 260, a loop filter 270 and a voltage controlled oscillator (VCO) 280. The CDR circuit 170 may include, for example, two inputs 290, 300. The first input 290 may provide, for example, a data signal received from the amplifier 140. The second input 300 may provide, for example, a reference clock signal. The CDR circuit 170 may include, for example, two outputs 190, 200. The first output 190 may provide, for example, a retimed data signal. The second output 200 may provide, for example, a clock signal. The first input 290 of the CDR circuit 170 may be coupled to respective inputs of the data retimer 240 and the phase detector 250. The second input 300 of the CDR circuit 170 may be coupled to an input of the frequency detector 260. A loop 350 may provide a feedback loop from an output 345 of the VCO 280 to respective inputs of the data retimer 240, the phase detector 250 and the frequency detector 260. An output of the data retimer 240 may coupled to the first output 190 of the CDR circuit 170. A first output 310 and a second output 320 of the phase detector 250 may be coupled to a first input and a second input of the loop filter 270. The first output 310 of the phase detector 250 may provide, for example, an error signal. The second output 320 of the phase detector 250 may provide, for example, a reference signal. An output 330 of the frequency detector 260 may be coupled to a third input of the loop filter 270. The loop filter 270 may be coupled to the VCO 280 via an output 340. The output 340 may provide, for example, a control signal such as, for example, a control voltage signal or a tuning signal. The output 345 of the VCO 280 may be coupled to the second output 200 of the CDR circuit 170 and may be coupled to the loop 350. The VCO 280 may be coupled to the data retimer 240, the phase detector 250 and the frequency detector 260 via the loop 350.

[0022] Although illustrated as separate components, the present invention also contemplates different levels of integration. For example, the phase detector 250 may be integrated, at least in part, with the frequency detector 260. In addition, although many of the signals are illustrated as single-ended signals, the present invention also contemplates that some signals may be represented as differential signals. For example, the reference clock signal provided on the second input 300 of the CDR circuit 170 may be a single-ended signal provided on a single line or a differential signal provided on at least two lines. Other examples of signals that may be represented as single-ended signals or differential signals include, for example, the data signal, the error signal and the reference signal.

[0023] In operation, a startup of the CDR circuit 170 may be initiated, for example, by turning on a power supply, receiving a valid link via the receiver 100 or other events or conditions. A reference clock signal (e.g., a system clock signal) may be provided at the second input 300 of the CDR circuit 170. The reference clock signal may be, for example, a relatively low-frequency signal generated by a stable oscillation source (e.g., a crystal). The frequency detector 260 may compare the reference clock signal with the output 345 of the VCO 280, which may be carried via the loop 350 to an input of the frequency detector 260. In one example, the output 345 of the VCO 280 may be divided down in frequency by, for example, a divider before being compared in the frequency detector 260 with the reference clock signal (e.g., a low-frequency system clock). The signal comparison may include, for example, calculating a difference signal. The frequency detector 260 may provide an output signal, which may be a function of the signal comparison. The output signal of the frequency detector 260 may then be filtered by the loop filter 270.

[0024] The loop filter 270 may provide a control signal (e.g., a voltage control signal for controlling the VCO 280) on output 340. The control signal may be used by the VCO 280 to adjust, for example, the frequency of the output 345 of the VCO 280 and thus, the output 200 (e.g., a clock signal) of the CDR circuit 170. If the frequency of the signal on the output 345 of the VCO 280 is too high, then loop filter 270 may provide a control signal on the output 340 such that the VCO 280 may lower the frequency of its output. If the frequency of the signal on the output 345 of the VCO 280 is too low, then the loop filter 270 may provide a control signal on the output 340 such that the VCO 280 may increase the frequency of its output. Since the loop 350 may provide the output 345 of the VCO 280 to the frequency detector 260 as a feedback signal, the frequency of the output 345 of the VCO 280 may be adjusted with greater precision until, for example, a particular threshold condition is satisfied.

[0025] The phase detector 250 may operate concurrently or sequentially with the frequency detector 260. In one embodiment, for example, the frequency detector 230 may first tune the output 345 of the VCO 280 to approximately the desired frequency before the phase detector 250 activates. The phase detector 250 may receive, for example, a data signal at a first input and may receive, for example, the output 345 of the VCO 280, via the loop 350, at a second input. The data signal may be carried, for example, on the first input 290 of the CDR circuit 170 which may be coupled to the first input of the phase detector 250. In one example, the output of the amplifier 140 may be coupled to the CDR circuit 170 and may provide the data signal to the first input 290 of the CDR circuit 170. The phase detector 250 may adjust the frequency and/or the phase of the output 345 of the VCO 280 by comparing the output signal carried by the output 345 of the VCO 280 and the data signal. In one embodiment, the phase detector 250 may determine a phase relationship between the VCO 280 output signal and the data signal. For example, the phase detector 250 may compare transitions in the data signal to the rising edges or the falling edges of a clock signal provided by output 345 of the VCO 280. The phase detector 250 may produce an error signal on the first output 310 of the phase detector 250 that may be proportional to the phase relationship. The phase detector 250 may also produce a reference signal on the second output 310 of the phase detector 250. The reference signal may be, for example, subtracted from the error signal to generate a correction signal that is independent of the data signal pattern, but is dependent on the phase error.

[0026] The loop filter 270 may then subtract and filter the error signal and the reference signal in generating a control signal (e.g., a voltage signal) on the output 340 of the loop filter 270. If the data signal provided by the first input 290 and the clock signal provided by the output 345 of the VCO 280 do not have the desired phase relationship, then the control signal may be used to correct the phase error. For example, if the data signal comes too soon (i.e., the data signal is advanced in time relative to the clock signal), then the phase detector 250 may increase the error signal (e.g., an error voltage signal) on the second output 310 of the phase detector 250. This may result in a change in the control signal on the output 340 which may, in turn, increase the frequency of the output signal (i.e., the clock signal) on the output 345 of the VCO 280. As the frequency of the clock signal increases, its edges come sooner in time (i.e., the edges advance in time). Thus, for example, the rising edges of the clock come in better alignment with the transitions or other reference points in the data signal. The feedback may insure that the data signal and the clock signal have the desired phase relationship for retiming the data via the data retimer 240. When the desired phase relationship is reached via the feedback, then the loop may be deemed locked. Thus, the CDR circuit 170 may include one or more phase-locked loops.

[0027] The data retimer 240 may generate retimed data. The data retimer 240 may receive, for example, the data signal provided by the first input of the CDR circuit 170 at a first input of the data retimer 240 and may receive, for example, the clock signal provided by the output 345 of the VCO 280 at a second input of the data retimer 240. The data retimer 240 may generate retimed data using, for example, the data signal and the clock signal. The output of the data retimer 240 may be coupled, for example, to the first output 190 of the CDR circuit.

[0028]FIG. 3 shows a diagram illustrating an embodiment of a phase detector according to the present invention. The phase detector 250 may include, for example, a delay block 350, a first sequential device 360, a second sequential device 370, a third sequential device 380, a first exclusive OR (XOR) gate 390 and a second XOR gate 400. In one embodiment, the first sequential device (FSD) 360 may be, for example, a flip flop; and the second sequential device (SSD) 370 and the third sequential device (TSD) 380 may be, for example, latches. In one example, the flip flop may be a negative-edge triggered device (i.e., the flip flop may change state on the falling edges of the clock signal). The latch may be adapted to pass data when its clock input is high and to latch data when its clock input is low.

[0029] A data input 410 to the phase detector 250 may be coupled to an input of the delay block 350 and to an input to the FSD 360. The data input 410 may be coupled, for example, to a first input of the phase detector 250 which, in turn, may be coupled to the first input 290 of the CDR circuit 170. The output of the FSD 360 and the output of the delay block 350 may be coupled to respective inputs of the first XOR gate 390. The output 430 of the first XOR gate 390 may provide, for example, an error signal that may be carried by the first output 310 of the phase detector 250. The output of the FSD 360 may be coupled to an input of the SSD 370. The output of the SSD 370 may be coupled to an input of the TSD 380. The clock input 420 may be coupled to the FSD 360, the SSD 370 and the TSD 380 and may provide a clock for each of the sequential devices. In one example, the TSD 380 may use the inverted clock signal as a clock signal. The clock input 420 may be coupled, for example, to a second input of the phase detector 250 which, in turn, may be coupled to the output 345 of the VCO 280 via the loop 350. The output of the SSD 370 and the output of the TSD 380 may be coupled to respective inputs of the second XOR gate 400. The output 440 of the second XOR gate 400 may provide, for example, a reference signal that may be carried by the second output 320 of the phase detector 250.

[0030] In operation, a data signal may be provided by the data input 410 and received by the FSD 360 and the delay block 350. The FSD 360 may be, for example, a flip flop. The flip flop may be clocked by a clock signal provided by the clock input 420. The clock signal may have been generated, for example, by the VCO 280. For example, on each falling edge of the clock signal, data in the data signal may be latched by the flip flop and may be held at its output. In one embodiment, the output signal of the flip flop may be the data signal retimed to follow a falling edge of the clock signal. The SSD 370 and the TSD 380 may be, for example, latches. The output signal of the flip flop may be passed by the latch of the SSD 370 when the clock signal is high or may be latched when the clock signal is low. In one embodiment, the output signal of the latch of the SSD 370 may be the output signal of the flip flop delayed by half a clock cycle. The output signal of the latch of the SSD 370 may be passed by the latch of the TSD 380 when the clock signal is low or may be latched when the clock signal is high. In one embodiment, the output signal of the latch of the TSD 380 may be the output signal of the latch of the SSD 370 delayed by half a clock cycle.

[0031] The delay block 350 may delay the data signal received from the data input 410 by a particular time delay. In one embodiment, the delay through the delay block 350 may be approximately equal to the clock-to-output delay of the flip flop of the FSD 360. The clock-to-output delay for a flip flop may be the delay of the output changing in response to a clock edge. The first XOR gate 400 may produce an output signal on the output 430 of the first XOR gate 400. The output signal of the first XOR gate 390 may be a function of the exclusive OR between the output of the delay block 350 and the output of the flip flop 360. The output signal of the first XOR gate 390 may be an error signal. The second XOR gate 400 may produce an output signal on the output 440 of the second XOR gate 400. The output signal of the second XOR gate 390 may be a function of the exclusive OR between the output of the latch of the SSD 370 and the output of the latch of the TSD 380. The output signal of the second XOR gate 390 may be a reference signal.

[0032]FIG. 4 shows a diagram illustrating an embodiment of a phase detector according to the present invention. The phase detector 250 may include many of the same components as described above including, for example, the FSD 360, the SSD 370, the TSD 380, the first XOR gate 390 and the second XOR gate 400. In FIG. 4, the delay block 350 is illustrated as a delay cell 450. The phase detector 350 may also include, for example, a first delay element 460, a second delay element 470, a third delay element 480 and a fourth delay element 490. The first delay element 460 may be disposed between the output of the delay cell 450 and the first input of the first XOR gate 390. The second delay element 470 may be disposed between the output of the FSD 360 and the second input of the first XOR gate 390. The third delay element 480 may be disposed between the output of the SSD 370 and the first input of the second XOR gate 400. The fourth delay element 490 may be disposed between the output of the TSD 380 and the second input of the second XOR gate 400. As illustrated, a delay element has been disposed in each of the four data paths. However, a delay element need not be present in each data path. Each delay element 460, 470, 480, 490 may be set to a particular delay time. The delay setting for a particular delay element may be changed. In one example, the delay settings may be programmable.

[0033] Although illustrated as separate components, the present invention also contemplates various degrees of integration. For example, the first delay element 460 may be integrated, at least in part, with the delay cell or with the first XOR gate 390. Similarly, the other delay elements 470, 480, 490 may be integrated, at least in part, with the sequential devices 360, 370, 380 or with the XOR gates 390, 400. Moreover, although illustrated as singled-ended signals, at least some of the signals may be represented as differential signals such as, for example, the error signal, the reference signal, the clock signal and the data signals carried on the four data paths. A differential signal may employ at least two lines, whereas a single-ended signal might employ only a single line. A delay element may be present in at least one of the lines carrying a differential signal in a data path.

[0034]FIG. 5 shows a diagram illustrating an embodiment of a delay element according to the present invention. Delay elements may be realized in any number of conventional manners. In one example, the delay element may include, for example, a variable capacitor 500. The variable capacitor 500 may be disposed between electrical ground and the line to which the delay element may be adding delay. The variable capacitor 500 may be formed in any number of conventional manners. In one example, the variable capacitor 500 may include conventionally configured transistors.

[0035] In operation, the delay elements 460, 470, 480, 490 may provide delays (e.g., different delays) which may, in turn, generate different phase-locking points (e.g., phase offsets) between the data signal and the clock signal. In one example, delay elements 460, 470 may provide delays to each of the inputs of the first XOR gate 390; and delay elements 480, 490 may provide delays to each of the inputs of the second XOR gate 400. By using a delay element for each branch input to the XOR gate 390, a wider range of delay mismatches may be available. However, a delay element need not be present in each branch input to the XOR gates. In one example, a relatively small amount of delay in each of the input branches of the XOR gates may generate a relatively large amount of relative delay between the data signal and the clock signal. In some applications such as, for example, some applications related to high-speed data recovery, introducing small delays in the data signal may help in preserving data integrity.

[0036] In some applications, the phase detector may lock the phase relationship between the clock signal and the data signal such that the latching clock edge is at the center between the data transitions. However, the data signal may be severely distorted and dispersed due to, for example, the data signal traveling over very long distances. The data signal may be subject to, for example, an asymmetrical jitter distribution. In such cases, the optimal phase relationship between the clock signal and the data signal may not be with the latching clock edge centered between the data transitions. Thus, through the use of the delay elements 460, 470, 480, 490, the locking point in the phase relationship between the clock signal and the data signal may be shifted (i.e., a particular phase offset chosen) so as to, for example, minimize the bit error rate (BER) or to provide optimum data regeneration.

[0037] One or more embodiments according to the present invention may have one or more of the advantages as set forth below.

[0038] By using delay elements to generate a different phase-lock position, the CDR circuit 170 may not accumulate charge. In one example, when no data is being received or when a long chain of ones or zeroes is being received, the CDR circuit may not accumulate charge. The accumulation of charge may have the potential of pushing the loop out of lock. Compare, e.g., “Method and Apparatus for High Speed Signal Recovery,” U.S. patent application Ser. No. 10/159,788 to Momtaz et al., assigned to the same assignee as the present application and filed on May 30, 2002. The above-referenced application is hereby incorporated by reference in its entirety.

[0039] One or more embodiments according to the present invention may generate the desired phase-offset from inside of the phase detector and the amount of phase-offset does not change with the input data pattern.

[0040] One or more embodiments according to the present invention may be used in, for example, linear-type phase detectors, binary-type (e.g., bang-bang type) phase detectors and other types of phase detectors.

[0041] According to one or more embodiments of the present invention, since the delay elements are added on the data path, the delay elements may be effective when there is a transition. Accordingly, the phase detector may only adjust the phase relationship between the clock signal and the data signal when the data signal is changing. Thus, the phase adjustment amount may be independent of the input data pattern.

[0042] While the present invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the present invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present invention without departing from its scope. Therefore, it is intended that the present invention not be limited to the particular embodiment disclosed, but that the present invention will include all embodiments falling within the scope of the appended claims. 

What is claimed is:
 1. A phase detector, comprising: a delay block coupled to a first exclusive OR (XOR) gate via a first delay element; a first sequential device coupled the first XOR gate via a second delay element; a second sequential device coupled to the first sequential device and coupled to a second XOR gate via a third delay element; and a third sequential device coupled to the second sequential device and coupled to the second XOR gate via a fourth delay element.
 2. The phase detector according to claim 1, wherein the first delay element comprises a variable capacitor.
 3. The phase detector according to claim 2, wherein the variable capacitor comprises a transistor.
 4. The phase detector according to claim 1, wherein the first sequential device comprises a flip flop.
 5. The phase detector according to claim 1, wherein the second sequential device comprises a latch.
 6. The phase detector according to claim 1, wherein the third sequential device comprises a latch.
 7. The phase detector according to claim 1, wherein the delay block comprises a delay cell.
 8. The phase detector according to claim 1, wherein the delay block is adapted to incorporate a time delay that is approximately equal to the clock-to-output time delay of the first sequential device.
 9. The phase detector according to claim 1, wherein a clock signal with an adjustable frequency is coupled to the first sequential device and to the second sequential device.
 10. The phase detector according to claim 9, wherein an inverse of the clock signal with the adjustable frequency is coupled to the third sequential device.
 11. The phase detector according to claim 1, wherein a particular phase offset between a clock signal and a data signal may be locked by setting a particular time delay for the first delay element, the second delay element, the third delay element and the fourth delay element.
 12. The phase detector according to claim 11, wherein the particular time delay for each of the delay elements is different.
 13. The phase detector according to claim 1, wherein a locked phase relationship between a clock signal and a data signal is adjusted by adjusting a time delay in at least one of the first delay element, the second delay element, the third delay element and the fourth delay element.
 14. The phase detector according to claim 1, wherein the phase detector only adjusts a phase relationship between a clock signal and a data signal when the data signal is in transition.
 15. The phase detector according to claim 1, wherein an amount of phase offset locked in between the clock signal and the data signal is independent of data signal pattern.
 16. The phase detector according to claim 1, wherein the phase detector is part of a clock and data recovery circuit.
 17. The phase detector according to claim 16, wherein the clock and data recovery circuit does not charge or discharge current in its phase adjustment loop.
 18. The phase detector according to claim 1, wherein the first delay element comprises a programmable time delay.
 19. A system for adjusting a locked phase offset between a data signal and a clock signal, comprising: a phase detector in which one or more data paths used in generating an error signal or a reference signal comprise a respective delay element; a loop filter coupled to the phase detector; and a voltage controlled oscillator coupled to the loop filter and an output of the voltage controlled oscillator coupled to an input of the phase detector.
 20. The system according to claim 19, wherein every data path used in generating the error signal or the reference signal comprises the respective delay element.
 21. A method for adjusting a locked phase offset between a data signal and a clock signal, comprising: adjusting a time delay in a respective delay element disposed in each data path of a phase detector.
 22. The method according to claim 21, further comprising: adjusting the locked phase offset between the data signal and the clock signal independent of data signal pattern.
 23. The method according to claim 21, further comprising: adjusting phase relationship between the data signal and the clock signal only during a transition in the data signal.
 24. A method for adjusting a locked phase offset between a data signal and a clock signal, comprising: adjusting a first time delay in a first data path of a phase detector; and adjusting a second time delay in a second data path of the phase detector. 